The need to eliminate lead-based octane enhancers in gasoline has provided an incentive for the development of processes to produce high octane gasolines blended with lower aliphatic octane boosters. Supplementary fuels are being examined by the petroleum refining industry. Lower molecular weight alcohols and ethers, such as isopropyl alcohol (IPA) and diisopropyl ether (DIPE), are in the boiling range of gasoline fuels and are known to have a high blending octane number. They are also useful as octane enhancers. In addition, by-product propylene from which IPA and DIPE can be made is usually available in a fuels refinery, typically as a C.sub.3 aliphatic stream which is rich in both propylene and propane.
In the past, IPA and DIPE were produced using the so-called indirect hydration processes. In the indirect hydration process, a selected olefin feed is absorbed in a concentrated sulfuric acid stream to form an extract containing the corresponding alkyl ester of the sulfuric acid. Thereafter, water is admixed with the ester-containing extract to hydrolyze the ester and to form the desired alcohol and ether which are then recovered, generally by stripping with steam or some other heating fluid. A diluted sulfuric acid stream is thereby produced. This acid stream is then generally treated to concentrate the sulfuric acid stream for recycle to the absorption stage.
In the indirect hydration process, the use of sulfuric acid as a catalyst presents certain problems. First, severe corrosion of process equipment can occur. Second, separating the produced ether from the sulfuric acid can be difficult. Third, a substantial quantity of waste sulfuric acid is produced in the concentration of the catalyst for recovery. Because of these problems, it has been found that the process of synthesizing DIPE by using concentrated sulfuric acid is not commercially viable. Clearly, there was a need for a more direct manner of bringing about the hydration reaction.
This need was addressed by so-called direct hydration processes using solid catalysts. In the direct hydration process, an olefinic hydrocarbon such as propylene is reacted directly with water over a solid hydration catalyst to produce an intermediate IPA stream from which the product DIPE can be formed. Development work using direct hydration focuses on the use of solid catalysts such as active charcoal, clays, resins and zeolites. Examples of olefin hydration processes which employ zeolite catalysts as the hydration catalyst can be found in U.S. Pat. Nos. 4,214,107, 4,499,313, 4,857,664 and 4,906,787.
The use of zeolites as hydration catalysts has the disadvantage of being expensive in comparison to other catalysts, for example, ion exchange resin catalysts. Also, in comparison to ion exchange resin catalysts zeolites do not operate as well at the relatively low temperatures required for hydration and etherification. Furthermore, zeolites have a strong tendency to form DIPE from reaction (2) instead of reaction (1). They also have a strong tendency to produce substantial amounts of undesirable polygasoline from the reaction of propylene with itself. ##STR1##
The preparation of DIPE from propylene can proceed by multiple chemical reaction sequences. Depending on the sequence used, the cost associated therewith can be a problem. In one sequence, propylene is hydrated to form IPA which is reacted with additional propylene to form diisopropyl ether. The hydration of propylene to form IPA is a difficult reaction to perform because, in their normal states, propylene is a gas and water is a liquid. Accordingly, the reaction of the two phases requires severe conditions, for example, a pressure of 1000 psig. Further, since the solubility of propylene in water is poor, severe conditions such as a pressure of 1000 psig-1500 psig are required to make this reaction work effectively. As a result, the hydration of propylene to form IPA is expensive.